Question

I just started taking physics online and I am stuck already. I
havenot taken physics in 2 years and I am already showing signs
ofstruggle. I tried working backwards on this equation given the
factthat I know the initial and final Q for W and using Q =
1/2(Q1+Q2) but I am falling short somewhere. Can someone please
helpme??

Here is the question:

Figure 21-36 shows four identical conducting spheres that
areactually well separated from one another.

Sphere W (with an initial charge of zero) is touched to sphere Aand
then they are separated. Next, sphere W is touched to sphere B(with
an initial charge of -32e) and then they are separated.Finally,
sphere W is touched to sphere C (with an initial charge of+48e),
and then they are separated. The final charge on sphere W is+18e.
What was the initial charge on sphere A?